Flatpak: Allow specifying the path to a destination repository

This makes the Flatpak builds honor the $FLATPAK_REPO environment variable.
When it is defined, it will be used as the path to an OSTree repository where
the Flatpak application bundle will be committed to. This allows to place
multiple builds in the same repository (along other applications, even), which
can be published. For more details abuit using repositories for publishing,
check:

   http://flatpak.org/developer.html#Distributing_Applications

Note that setting $FLATPAK_REPO disables building a standalone .flatpak bundle
file.

build/gulpfile.vscode.linux.js

@@ -215,8 +215,13 @@ function buildFlatpak(arch) {
 	];
 	const buildOptions = {
 		arch: flatpakArch,
-		bundlePath: manifest.appId + '-' + flatpakArch + '.flatpak',
 	};
+	// If requested, use the configured path for the OSTree repository.
+	if (process.env.FLATPAK_REPO) {
+		buildOptions.repoDir = process.env.FLATPAK_REPO;
+	} else {
+		buildOptions.bundlePath = manifest.appId + '-' + flatpakArch + '.flatpak';
+	}
 	// Setup PGP signing if requested.
 	if (process.env.GPG_KEY_ID !== undefined) {
 		buildOptions.gpgSign = process.env.GPG_KEY_ID;